Reference:
https://www.cisco.com/c/en/us/td/docs/security/firepower/60/configuration/guide/fpmc-config-guide-v60/Applic

Explanation
The default action setting in a Cisco FTD Access Control Policy determines how the system handles and logs traffic that is not handled by any other access control configuration. The default action can block or trust all traffic without further inspection, or inspect traffic for intrusions and discovery data3.
The Trust All Traffic option allows all traffic from an undefined application to pass without Snort inspection.
This option also disables Security Intelligence filtering, file and malware inspection, and URL filtering for all traffic handled by the default action. This option is useful when you want to minimize the performance impact of access control on your network3.
The other options are incorrect because:
The Inherit from Base Policy option inherits the default action setting from the base policy. The base policy is the predefined access control policy that you use as a starting point for creating your own policies. Depending on which base policy you choose, the inherited default action setting can be different3.
The Network Discovery Only option inspects all traffic for discovery data only. This option enables Security Intelligence filtering for all traffic handled by the default action, but disables file and malware inspection, URL filtering, and intrusion inspection. This option is useful when you want to collect information about your network before you configure access control rules3.
The Intrusion Prevention option inspects all traffic for intrusions and discovery data. This option enables Security Intelligence filtering, file and malware inspection, URL filtering, and intrusion inspection for all traffic handled by the default action. This option provides the most comprehensive protection for your network, but also has the most performance impact3.

Integrated routing and bridging (IRB) is a feature that allows you to route between different bridge groups on a Cisco FTD device. A bridge group is a logical interface that acts as a container for one or more physical or logical interfaces that belong to the same layer 2 broadcast domain. You can assign an IP address to a bridge group interface (BVI) and enable routing protocols on it, just like a regular routed interface. However, when you run a dynamic routing protocol on a Cisco FTD device in IRB mode, you can only use nonbridge interfaces as routing peers. You cannot use bridge group interfaces or bridge group member interfaces as routing peers2. This is because the routing protocol packets are sent and received on the nonbridge interfaces, and the bridge group interfaces are used only for forwarding data traffic3.